Preheat your oven to 425°F (220°C) and grease four ramekins with unsalted butter, dusting the insides with cocoa powder.

Melt semi-sweet chocolate and unsalted butter in a heatproof bowl over a double boiler, stirring until smooth.

In a separate bowl, whisk together one large egg, one egg yolk, and confectioners' sugar until pale and fluffy, then add a splash of vanilla extract.

Carefully combine the melted chocolate mixture with the egg mixture, whisking gently to incorporate without cooking the eggs.

Sift in all-purpose flour, cocoa powder, and sea salt, folding gently until no dry ingredients remain.

Divide the batter among the ramekins, filling each about three-quarters full, and place them on a baking tray.

Bake for 12-14 minutes or until edges are firm but centers jiggle slightly, then let cool for 1 minute before inverting onto plates.